The invention relates to regenerators for closed thermodynamic cycle coolers. More particularly, the invention relates to low temperature regenerators for cryogenic coolers.
In closed cycle coolers, a single quantity of working gas is repeatedly used in the thermodynamic cycle. In cryogenic coolers, the working gas chosen is usually helium. This choice is made because helium has the lowest liquefaction temperature of all known gases.
Examples of closed thermodynamic cycles are the Stirling cycle and the Vuilleumier cycle, both of which use regenerators. In these thermodynamic cycles, the regenerator functions as a near approximation to a heat reservoir. That is, the regenerator can reversibly store and release a given quantity of heat with minimal change in its temperature. The quantity of heat for which this holds true is a matter of design for each individual cooler application.
In the design of a regenerator, many different factors must be considered. For example, the rate of heat conduction of the regenerator material must be sufficient to assure that the regenerator stores and releases the desired quantity of heat in the time it takes for the working gas to pass through the regenerator. To achieve this requirement, it is generally preferred to choose a material with a high heat conductivity. At the same time, however, the thermal conductivity along the length of the regenerator, in the direction of working gas flow, should be low because there is usually a substantial temperature difference which must be maintained between the ends of the regenerator. In addition, the surface area of the material should be maximized to maximize contact with the working gas, and the sizes of individual particles of the material should be minimized to decrease the distance the heat must travel within the material.
On the other hand, another factor to be considered in designing a regenerator is the pressure drop in the working gas as it passes through the regenerator. The higher the pressure drop, the lower the efficiency of the cooler. However, when maximizing heat conduction by minimizing particle size, as discussed above, the result is an increase in the pressure drop in the working gas. Hence, a suitable compromise between these competing factors must be chosen.
Another important physical property to be considered in designing a regenerator is the heat capacity of the regenerator. The higher the heat capacity, the closer the regenerator approximates a heat reservoir. One way of increasing the heat capacity of a regenerator is to increase the mass of the regenerator. However, this method is typically limited by size, and weight, and dead volume constraints. Another method of increasing the heat capacity of a regenerator is to choose a material having as large a heat capacity as possible.
When designing a regenerator for a cryogenic cooler, special problems arise. For most materials the heat capacity drops as the temperature of the material drops. Accordingly, the heat capacity of the regenerator at the coldest working temperature of the regenerator is an important design consideration. Due to this known material limitation, closed cycle cryogenic refrigerators, such as those operating on the Stirling cycle, have in the past been limited in their ability to attain temperatures below about 10.degree. K. The heat capacity of the regenerator material, usually lead, becomes so small at this temperature that the regenerator efficiency then plummets and the net cooling capacity of the cryogenic cooler quickly approaches zero.
In addition, the efficiency of regenerators at slightly higher temperatures (such as 20.degree. K.) is characteristically low for the same reason; the heat capacity of the regenerator material is very small even at these temperatures. As a result, the overall cooling efficiency suffers.